A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Ninja Open: Create a battlefield (not at any Province). Assign Kumoru to attack there and another player's target unbowed Personality to defend there. Fight a battle there. Battle resolution there will not destroy Personalities. Lose 2 Honor.
a) Does this prevent the defender from assigning any more Personalities? If so, please explain why.
b) Can both players Cavalry engage?
Ninja Open: Create a battlefield (not at any Province). Assign Kumoru to attack there and another player's target unbowed Personality to defend there. Fight a battle there. Battle resolution there will not destroy Personalities. Lose 2 Honor.
a) Does this prevent the defender from assigning any more Personalities? If so, please explain why.
b) Can both players Cavalry engage?
a) No other Personalities may assign. Assignment is done via a card effect, which in this case is just Kumoru and the target, or via the rulebook, which lets you assign Personalities to battles as part of an Attack Phase. Fighting the battle is an effect of Kumoru's action: it's not caused by the rulebook; none of the steps of an Attack Phase occur outside an Attack Phase unless a card effect says they do.
b) Yes. The Cavalry ability is an Engage ability which may be used during the battle's Engage Segment as normal - although if you do, you obviously can't then use it in later battles that turn.